This invention relates to a new and improved green ceramic article which can be fired to produce an improved suppressor for use at elevated temperatures.
Suppressor elements suitable for use in spark plugs must have good mechanical and electrical stability at high temperatures, a wide operating temperature range, uniform resistance value and good suppression of high frequency oscillations associated with spark discharge in ignition systems.
The problem of eliminating radio frequency radiation from the high voltage ignition system of internal combustion engines has been of increasing concern in recent years because such radiation produces interference with the use of radio channels for communication and navigation. This problem has been accentuated by the increasing number of automobiles, boats and aircraft and the simultaneous increase in the use of radio frequency equipment in both communications and navigational equipment.
The typical ignition system for an internal combustion engine includes a set of breaker points, a capacitor, an ignition coil, a spark plug, and connection wires. When the breaker points are closed, a battery causes a current to flow in a primary winding of the ignition coil, thereby establishing a magnetic field about, and storing energy in, a ferrous core in the ignition coil. When the breaker points are opened, the magnetic field collapses and produces a high voltage across a secondary winding of the ignition coil. The high voltage is applied to, and arcs across, a spark gap in the spark plug, greatly decreasing the impedance of the gap. The secondary coil winding and the low impedance spark gap form a resonant circuit which oscillates as the energy stored in the core is dissipated. The oscillations are in the radio frequency spectrum and may cause severe noise and interference in both communications equipment and navigational equipment.
In the past, it has been found that random radio frequency radiation from the ignition system of internal combustion engines may be greatly reduced or eliminated by placing a resistance element in the high voltage ignition circuit for each spark plug. The resistance element may be positioned in the bore of a spark plug insulator, in series with the spark plug center electrode, or may be placed at some other convenient location in the ignition system, such as in a distributor rotor or distributed in the high voltage ignition cables.
Prior art suppressors, other than distributed resistances found in ignition cables, are generally either of a carbon rod type, of a wire wound type, of a sintered resistive rod type or of a resistive mass fired between the glass seals in the center electrode bore through a spark plug insulator. Each of the different types of suppressors has advantages and disadvantages. The carbon capsule suppressor is, for example, relatively inexpensive compared to a wire wound suppressor. The carbon capsule usually consists of carbon or graphite dispersed in a resinous binder. However, when the carbon capsule suppressor is placed in a spark plug and is heated to perhaps 450.degree.F. or more during operation of the internal combustion engine, the carbon tends to oxidize, resulting in an open circuit due to rapidly increasing resistance levels as the carbon oxidizes, until a value of infinity is reached. Vitreous type carbon suppressor elements, formed from clay, talc and a refractory material having carbon distributed therein, have been used extensively. However, it is difficult to prepare such suppressors having uniform resistance values.
Wire wound suppressors do not possess as high a resistance level as carbon resistors because they suppress by inductive impedance rather than by resistance impedance. However, the wire wound suppressor is expensive compared to the carbon resistor and presents problems both in arcing and in connecting terminals to the wire ends. Wire wound suppressors are also bulky and, therefore, difficult to use in smaller size spark plugs.
Suppressor elements suitable for use in an internal combustion engine must withstand severe operating conditions involving pulsating high power loadings. The suppressor element must operate well at temperatures ranging from 200.degree. to greater than 400.degree.F. at 15,000 volts pulsating direct current.
In an attempt to overcome difficulties encountered with the use of carbon, other suppressor composition systems have been suggested. For example, U.S. Pat. Nos. 2,864,773 and 2,969,582 disclose the use of titanate and stano-titanate type materials modified to obtain desired electrical characteristics.
The Radio Manufacturers Association (RMA) and the Society of Automotive Engineers (SAE) have directed efforts toward determining limits for interference from internal combustion engines in communication and navigation equipment. As a result, the SAE has adopted limits for impulsive type interference and has included these limits in a uniform test standard SAE J551b, "Measurement of the Vehicle Radio Interference".
It is known that significant improvements can result in operation of communication and navigation equipment when engine-driven apparatus comply with the limits set forth in SAE J551b. Communications apparatus that operate in the frequency range 20-1000 megahertz which might be susceptible to radio frequency interference are very high frequency (VHF) television, ultra high frequency (UHF) television, frequency modulated (FM) radio, aircraft navigation and communication, amateur radio, telemetry, high frequency (HF) communications, UHF radar, and others.
The testing equipment required for SAE J551b is complex and expensive. However, satisfactory testing results can be obtained by comparing test samples with a wire wound suppressor and a carbon suppressor having known resistance and suppressing properties, and measuring the field intensity per unit band width within a given frequency range.
Manganese oxide and manganese-nickel oxide resistor elements are known in the art. However, such compositions have a relatively high temperature coefficient of resistance, which is undersirable in controlling radio frequency radiation.